Sylvan Elves versus Demons.

Against a ballista, light magic is effective – expert-level evasion provides good protection, plus demons often use dark magic, so I think light magic is essential. And for dealing with a hail of arrows, you don't necessarily need to invest in chaos magic; it's enough to simply find an ice spell. The damage from an ice spell without the chaos skill and with expert-level proficiency isn't significantly different. Here's what the manual says:

without skill = 60+12*L
basic = 72+12*L
advanced = 84+12*L
expert = 96+12*L

so, spending 3 levels for 36 damage isn't really necessary.